1. Field of the Invention
A process for the preparation of a K-A type zeolite, and separation by adsorption of mixtures using zeolite or a molecular sieve (Class 23, Subclass 113, and Class 55, Subclass 74.)
2. Description of the Prior Art
In the prior art, a variety of zeolites, natural and artificial, have been industrially used. Artificial zeolites are preferred because of their uniform properties, and a typical one is the A-type zeolite. A-type zeolite is represented by its sodium salt having the chemical composition of Na.sub.12 (AlO.sub.2.Si0.sub.2).sub.12 (NaAlO.sub.2).sub..delta.. XH.sub.2 O, wherein O.ltoreq..gamma..ltoreq.1, and X is an integer. Twelve sodium ions in the formula are chemically active and readily exchanged by other cations. Those A-type zeolites in which sodium ions are replaced by potassium ions and calcium ions are called K-A type zeolite and Ca--A type zeolite, respectively, and an unreplaced one is called Na--A type zeolite. The A-type zeolite varies in the size of pore openings for adsorption depending on the kind of cations exchanged. For instance, Na--A type zeolite, Ca--A type zeolite, and K-A type zeolite adsorb selectively molecules having diameters of 4A, 5A and 3A, or less respectively, and they are conventionally defined as 4A zeolite, 5A zeolite, and 3A zeolite. This characteristic is attributed to the crystal structure of A-type zeolite. That is, in the case of Na-A type zeolite, eight 6-membered oxygen rings and three 8-membered oxygen rings contained in the unit lattices are occupied by eight and four sodium ions, respectively, and the adsorption property is affected by the kind of ion which exchanges the four sodium ions in the 8-membered oxygen ring. The detailed physico-chemical mechanism of cation exchange by the A-type zeolite has not been elucidated.
The prior art is represented by U.S. Pat. Nos.
3,078,645 for: HYDROCARBON SEPARATION, and PA1 3,078,636 for: UNSATURATED HYDROCARBON SEPARATION both issued Feb. 26, 1963 to R. M. Milton, PA1 3,056,654 for: PROCESS OF MAKING CRYSTALLINE ZEOLITE K-G issued Oct. 2, 1962 to BARRER et al. and the present inventors own prior U.S. Pat. No. 3,785,122 of Jan. 15, 1974, entitled to the benefits of the same priorities as the present case.
Barrer et al corresponds to the rhombohedral crystaline system with a unit cell edge equal to 9.52A. The A-type zeolite used in the present invention differs substantially from the K-G zeolite in its characteristics. For better understanding of the difference, reference is made to "Silicate Science", Vol. IV, Academic Press, 1966, pages 500-501.
In addition Barrer et al recites a zeolite of substantially less metal content than of the instant invention, causing substantial changes in the molecular formula. Further Barrer et al does not recognize the expansion of the intracrystalline voids by exchange of calcium ion for sodium ion.
U.S. Pat. No. 3,078,645 discloses a prior art method which effects conversion from the Na--A type zeolite to the Ca--A type zeolite by exchange of about 20 - 40% of calcium.
U.S. Pat. No. 3,078,636 deals with separation of unsaturated hydrocarbon in which conventional A-type zeolites are used and teaches adsorption of saturated hydrocarbon and nitrogen, wherein the separation of the gas depends merely on discrepancy of the absorbility and it does not and can not utilize sieving action.